Wednesday, August 29, 2012

Why build a clock that displays four letter words? The current time is everywhere; on your PC, smartphone, GPS, MP3 player, etc. Heck, you may even own a watch! Four letter words are pretty common as well.

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So why then?
1st) I had this display that I bought from DealExtreme.Com. The only reason I got it was because it was so cheap.

2nd) I wanted to experiment with writing/reading data from an EEPROM with a microcontroller.

3rd) I wanted to experiment with controlling two devices on the I2C bus in one application.

But.... I wanted a fun project idea to make the effort seem somewhat worthwhile and settled on a Four Letter Word Clock.

If you you just want to see the results and are not interested in the build details, here is a short video demo.

The time is shown in 24 hour format on the left four 7-segments. Every second a different four letter word is shown on the right four 7-segments. The eight LEDs under the 7-segments progress from left to right as a way to display seconds. If you listen closely to the video and you can hear a relay that gives the clock a mechanical ticking sound.

The buttons under the LEDs are used to set hours (S1), minutes (S2), increase display brightness (S6), decrease display brightness (S7), and turn ON/OFF the mechanical ticking sound (S8) from the relay.

The display from DealExtreme.Com is pretty awesome for the price. It contains eight 7-segment LED displays, eight LEDs that can be red, green, or red/green, and eight button switches. The display has a solid, well built feel to it and was a bargain at $4.99. As a plus, you can control all these feature with only three I/O pins on a microcontroller. On the downside, it ships with no documentation (zero, zip, nada...) so plan on doing some web searching to understand it.

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The 24LC256 EEPROM, DS1307 RTC, and PICAXE 18M2 are easy to get from many web sources. I rescued the Teladyne 712-5 relay from a trash bound PCB. A good thing because a web search shows that relay at $28 (it's an RF spec relay!). No fear, you can leave the relay off or just use any cheap relay as it is not used to switch any current, just for the ticking sound.

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Now came the time to load the 24LC256 EEPROM with four letter words. So... to the internet for a quickie download of all the four letter English words (including all your favorite cuss words) in one tight ASCII text file. Unfortunately, 7-segment displays don't display letters like "K", "M", "V", "W", "X", and "Z" very readable. I wrote a short Python script to pull out the offenders, which also meant some of the more 'expressive' words where lost. After it was all done, there were 1,003 four letters words that easily fit into the 24LC256 EEPROM. A short (and separate) program was written to tell the PICAXE 18M2 to load these words into the 24LC256 EEPROM.

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The harder part was the code to drive the display. The lack of documentation made it pretty challenging. I always find the PICAXE forum helpful in these situations (special thanks to "mjy58"). After much coding/debugging, the problem was solved.

Controlling the two I2C devices (the 24LC256 EEPROM and the DS1307 RTC) from the PICAXE 18M2 was a bit easier than I expected after sorting through the addressing procedures.

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Here is a short vid (time lapse) of the rig working on the AXE091 development board. In the vid you can see the eight red LEDs progress from left to right as the seconds tick by.

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After verifying the operation for a few days the whole mess was moved off the AXE091 development board and onto a strip board PCB. Installing the rig into a $3.49 metal project box from Radio Shack provided a clean finished product.